ppp: add memmove fortify and remove MRU patch

ppp: add memmove fortify and remove MRU patch

memcpy() with overlapping src and dest buffers is an undefined behavior
in C. In the current code, a ConfRej response is generated by copying
input data in-place, where the dest address is lower than the src.
This happens to work in practice because memcpy() forward-copies data,
matching the behavior of memmove() in this case.

However, if FORTIFY_SOURCE or Address Sanitizer is enabled, memcpy()
will detect the overlap at run time and abort the program.

Replace the memcpy() with memmove() to ensure a well-defined behavior.

Reported-by: Filippo Carletti <filippo.carletti@gmail.com>
MRU patch https://github.com/ppp-project/ppp/pull/573

Signed-off-by: Paul Donald <newtwen+github@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/22286
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>